Goldfish and Koi are wonderful at providing nitrates and other nutrients for plants. I was also adding fertilizer tablets for the two lotus and lilies. So, when I was watering my orchids and other plants with this, they were really happy. Then I broke down my pond and began watering with rain water with orchid fertilizer added. I just assumed that a fertilizer advertised for orchids would have everything they needed, like the fertilizers for my other plants. Sigh.
I use red lava rock as a medium for my cattleyas so I just put the eggshells among the rock, not broken too much. I tried oyster shell but it went right through the rock. Eggshell doesn't work immediately (it must begin to decompose) but, later, provides a slow, steady release. As for the powdered milk, I add about a spoonful. Hope this helps. I've been doing this for the past three years and haven't had any trouble since.